Recently, from the viewpoint of global environment protection, there is an increasing demand for improved fuel efficiency in automobiles. Further, from the viewpoint of protecting occupants in vehicle collisions, there is another increasing demand for improved safety of automobile bodies. As such, to satisfy the needs for both improved fuel efficiency and enhanced safety, many considerations have been given toward the possibilities of achieving both reducing weight and reinforcing automobile bodies.
Stronger and thinner component materials are effective to satisfy both reducing weight and reinforcing automobile bodies. Lately, ultra high strength members using high tensile steel sheets having a tensile strength TS of 1180 MPa or more are beginning to be used as automobile framework members, reinforcing members, and so on.
However, as described in “Delayed Fracture,” Nikkan Kogyo Shimbun Ltd., Aug. 31, 1989, high strength steel sheets having TS of 1180 MPa or more are more likely susceptible to delayed fracture during use due to penetration of hydrogen associated with corrosion, as compared to other steel sheets having, lower strength. This limits application of such high strength steel sheets having TS of 1180 MPa or more.
Further, members such as automobile framework members are generally put into use after being subjected to forming such as press forming and roll forming. However, it is known that delayed fracture resistance of these members tends to deteriorate due to such forming process, as described in International Journal of Automotive Engineering, Vol. 39, No. 5, p. 133. Thus, there are demands for an ultra high strength member that is excellent in delayed fracture resistance after forming process.
On the other hand, when TS is equal to or higher than 1180 MPa, formability itself degrades.
Moreover, members such as automobile framework members are usually put into use after being subjected to first forming and then chemical conversion treatment and electrodeposition coating. Delayed fracture may occur due to penetration of hydrogen during chemical conversion treatment and electrodeposition coating in such cases. Although delayed fracture is less likely to occur during chemical conversion treatment and electrodeposition than in a corrosion environment in actual use, there is a possibility that delayed fracture may occur during chemical conversion treatment and electrodeposition coating, which are supposed to be milder than a corrosion environment in actual use, when strength of the member is increased to 1320 MPa or more in particular. Thus, it is necessary to prevent delayed fracture from occurring during the conversion treatment or electrodeposition coating after forming.
For example, known as one of the methods to solve this problem is a technique where a steel sheet is formed, while the steel sheet is hot and strength thereof is lowered, and simultaneously cooled in a die, so that a high component strength is obtained (this technique will be referred to as a “hot pressing process” hereinafter), as disclosed in Press Technology, Vol. 42, No. 8. p. 38 and UK 1490535. This hot pressing process is known to offer better delayed fracture resistance to workpieces than working at room temperature does because in the former: (1) no strain due to working remains, (2) residual stress due to working is small, and so on (see Press Technology, Vol. 42, No. 8, p. 38).
However, when automobile components are manufactured by the hot pressing process, some working steps are required, such as circumference trimming of components by punching and shearing for the purposes of shaping components after working, or perforation by punching which is necessitated by assembling purposes (these steps will be collectively referred to as “punching”). Such punching after the hot pressing process introduces a large strain and residual stress to the steel sheet, thereby significantly increasing the risk of delayed fracture during use. To solve this problem, the following two methods have been primarily considered:
(a) reducing the amount of hydrogen penetrating into a steel sheet during heating at the time of hot pressing; and
(b) reducing residual stress by punching after hot pressing.
Regarding (a) above, for example, JP-A 2006-104527, JP-A 2006-110713, JP-A 2006-111966 and JP-A 2008-284610 disclose techniques for reducing the amount of hydrogen penetrating into steel during heating by controlling the atmosphere in a heating furnace. Further, JP-B 4288201 discloses a technique for improving the resistance to delayed fracture susceptibility by heat treatment at 150 to 700° C. following hot pressing to release the hydrogen which has penetrated into a steel sheet during hot pressing.
Regarding (b) above, JR-A 2006-104527 discloses a technique for reducing the residual stress due to punching by reducing the cooling rate after hot pressing of a portion to be punched and thereby reducing the strength due to the resulting insufficient quenching.
Further, JP-A 2006-110713 discloses a technique for improving delayed fracture resistance by using a laser or plasma to melt, cut and remove any portion where the residual stress generated by punching remains.
JP-A 2006-111966 discloses a technique for improving delayed fracture resistance by removing any portion where the residual stress generated by punching remains by machining or the like.
JP-A 2008-284610 discloses a technique for improving delayed fracture resistance by precisely controlling the clearance of punching after hot pressing to reduce the ratio of shear droop length to sheet thickness.
Further, JP-A 2009-197253 discloses a technique for improving the resistance to delayed fracture susceptibility by performing heat treatment at 300° C. or higher but not higher than 400° C. for 10 minutes or less after punching and thereby reducing the tensile residual stress residing in a processed edge.